A ~250 KYR TEPHROSTRATIGRAPHY FROM THE DEEPCHALLA DRILLED LAKE RECORD: NEW CHRONOSTRATIGRAPHIC TIE-LINES BETWEEN PALAEOENVIRONMENTAL AND GEOARCHAEOLOGICAL SEQUENCES IN KENYA

Throughout the timeframe of hominin evolution, explosive volcanism along the length of the East African Rift (EAR) has intermittently blanketed landscapes with volcanic ash (tephra). Tephra layers in outcrops, or exposed at depth by excavation, have long provided a means to date and correlate geoarchaeological sequences. More recently, systematic investigations of tephra layers in lake-sediment records are filling in the gaps in regional tephrostratigraphies and provide an expanding dossier of geochemically characterised tephra isochrons that can be used for the correlation of diverse tephra-bearing records over distances of 10s to 1000s of km distance. An example is provided by the tephrostratigraphy of the ~250 kyr DeepCHALLA drilled lake-sediment record, which was retrieved in 2016 from Lake Chala, a crater lake near Mt Kilimanjaro on the border of Kenya and Tanzania. The DeepCHALLA sediment sequence provides a high-resolution palaeoenvironmental record contemporary with the Middle and Late Stone Age in eastern Africa.

Thirty visible tephra layers derive either from off-rift basaltic scoria-cone eruptions in the nearby Chyulu Hills and Mt Kilimanjaro, from explosive activity at Mt Meru ~100 km west of Chala, or from volcanoes further afield in the Central Kenyan Rift. Direct Ar-dating of 11 tephra layers, with suitable K-rich crystal contents, provide absolutely dated anchor points for the DeepCHALLA chronology. Geochemical correlations between visible tephra found in Lake Chala sediments and in geoarchaeological sequences across Kenya demonstrate the potential for using tephra isochrons to precisely link these records. Ongoing analysis of this sediment record indicates that the visible tephra layers represent only a fraction of those recorded in the Lake Chala sequence. Also present throughout the sequence are cryptotephra – microscopic layers of tephra not visible to the naked eye that derive from extremely far-travelled, or low volume, explosive eruptions. The DeepCHALLA tephra record is providing unprecedented detail into the timing and reach of past explosive volcanism in the Kenyan Rift. Furthermore, with both regional and extra-regional tephra tie lines to geoarchaeological sequences, the site is a key locality for refining an interconnected tephrostratigraphy of the EAR.